Grinding processes have been used in industrial processes, such as for cement and minerals production and for food processing. The purpose is to grind raw material to an adequate grain size. Thus, the surface is increased so that subsequent chemical reactions or mechanical separation processes can take place more efficiently.
A large amount of electrical energy is consumed in the grinding processes. For example electrical energy cost is the second largest cost position in the cement production process. For that reason grinding optimization systems are widely used in industry.
These optimization systems rely on the amount of mass inside the grinding unit which is related to a filling level or filling degree. This quantity changes continuously for instance due to the variable hardness of the material to be ground. Measuring inside a grinding unit is not possible during operation because of the grinding work and resulting dust. An additional issue is that an output from the grinding unit can not be measured directly because the mass transport is done by convection either with water or with air.
A measurement principle outside of the grinding unit, which is also known as an “electric ear” and which is widely used such as for ball mills, detects noise or vibration of a mill shell with a microphone. The idea is that when the mill is becoming empty, the noise or the vibration level is higher than when the mill is filling up. The results however can be unreliable and often useless when there is a poor accuracy of the measurements. Performing a spectral analysis of the noise and identifying a relative share of selected frequencies can slightly improve the accuracy. But an air gap between the mill shell and the microphone which can make the method very prone to interferences from neighboring sources of noise, and the thick walls of the shell which unpredictably influence different frequencies, can still lead to an insufficient accuracy.
DE 19933995 discloses a measurement system for observing a mass inside a ball mill using microphones directly attached to the wall of the mill shell. The system analyzes an intensity and spectra of the noise, and combines them with measurements of the phase angle of the mill to gain additional information about the movement of balls inside the mill. But the proposed solution can suffer from an unpredictable influence and an insufficient accuracy.